U.S. Pat. No. 4,042,661, issued Aug. 16, 1977, discloses a method and apparatus for forming tubular multilaminate plastic film. The apparatus incorporates a die body having two inlet passageways, each of which is adapted to receive a stream of resin. Each inlet passageway is in communication with an annular chamber. The annular chambers are substantially concentric and each communicates with an annular outlet passageway. The outlet passageways converge to form a single common annular passageway.
As each resin stream of the arrangement disclosed in U.S. Pat. No. 4,042,661 enters the die body and flows through the inlet passageway, it has a generally rectangular cross section. When it enters the annular chamber, it divides into two substreams which flow laterally in opposite directions to form, when they are joined together, a stream having an annular cross section. The respective annular streams flow from the annular chambers through the outlet passageways and converge where the outlet passageways converge to form a composite stream which flows through the common outlet passageway into a conventional die head from which the composite stream is expressed to form a tubular multilaminate film, which may be inflated with air to form blown film.
While the arrangement disclosed in U.S. Pat. No. 4,042,661 is highly satisfactory in many respects, some difficulties are encountered when the operator attempts to regulate the flow of the annular stream from the inner annular chamber. Access to the adjustment mechanism of the inner choke ring commonly employed for such purpose is difficult and proper adjustment can be a time consuming chore.
The present invention overcomes the aforesaid difficulties by utilizing a die component of the spiral mandrel type to form at least one of the sheets or laminae of an extruded multilaminate tubular film. U.S. Pat. No. 4,298,325, issued Nov. 3, 1981, discloses another version of a die for the extrusion of plastic material in tube form. In that patent, a plastic tube having a highly uniform wall thickness is formed. Because spiral manifolds are center fed and symmetrical, they normally require no adjustments for different resins or conditions. However, such devices are very unwieldy and not totally applicable for the formation of thin film layers.
The system of the present invention has the positive attributes of the two tubular multilaminate film formation techniques just described, without their disadvantages. According to the invention, a film is produced which has a base lamina or sheet of highly uniform thickness formed from spirally extruded resin and a multi-layered sheet bonded to the base sheet formed of a plurality of thin layers formed from non-spirally extruded resin.
Another advantage of the present invention is its utilization of a manifold and slit employed in the formation of the multi-layered sheet having shear rates substantially equal and constant. Both layer and thickness uniformity of the non-spirally extruded sheet are a consequence. While the general concept of making manifold and slit shear rates substantially equal and constant has been recognized as applicable to small flat dies (see the paper entitled Design of Dies for the Extrusion of Sheets and Annular Parisons: The Distribution Problem by H. H. Winter and H. G. Fritz published in ANTEC '84, pages 49-51), the present invention is the first to apply such principle to large blown dies, thus eliminating the need for choker rings or other adjustment arrangements. Furthermore, the paper just noted is concerned with monolayer extrusions and does not at all concern itself with coextruded films.